on British Palceozoic Crinoids. 285 



condition we have met with is shown in fig. 2 (PI. XVI.). 

 The calyx is covered by a round dome of oral plates, the 

 height of which relatively to that of the radial plates is 

 greater than in any other specimen we have seen. Its base 

 is very nearly as wide as the summit of the radial pentagon, 

 which is tlius almost entirely concealed. At the centre of the 

 upper edge of each radial is a minute opening, which pene- 

 trates beneath the dome ; but no arm-facet corresponding to 

 this opening is visible. The oral plates are so closely united 

 that there is no trace of the sutures between them, though 

 there are five faint grooves on the upper surface of the dome, 

 which indicate their median lines. In older specimens, figs. 4, 

 7, 8, 10 (PI. XVI.), these grooves are sometimes very marked. 

 In this youngest individual the calyx is tolerably symmetrical, 

 no one part being further developed than another. The same 

 is the case in some older individuals, as is shown in figs. 5 

 and 7 (PI. XVL). On the other hand, specimens are not 

 uncommon with some of the radials more developed than 

 others. The youngest stage in this condition that we have met 

 with is shown in fig. 1 (PI. XVL). The oral plates in this 

 specimen are relatively lower, but cover the radials more com- 

 pletely than in the original of fig. 2, and their median grooves 

 are much more distinct. Three of the radials have rudimen- 

 tary arm-facets ; but on the other two there is scarcely any 

 more indication of these structures than in the specimen re- 

 presented by fig. 2 (PL XVL). 



A larger and more advanced individual in the same condi- 

 tion is shown in fig. 3 (PI. XVL). The dome of oral plates 

 is remarkably flat ; and three of the radials have minute semi- 

 circular arm-facets, which are much less distinctly visible in 

 the two remaining radials. 



The next stage, in which the arm-facets are equally deve- 

 loped on all the radial plates, is exhibited in figs. 4-7 

 (PI. XVL). The calyx, which varies considerably in form, 

 is surmounted by a low rosette-like dome, composed of the 

 five very closely ankylosed orals. Each of these plates is 

 triangular in shape and excavated rather deeply along its 

 median line. At the centre of the dome they are in close 

 contact laterally, so that no opening is visible ; but their basal 

 angles are more or less truncated, leaving a superficial gap 

 between every pair of plates, which corresponds in position 

 with the articular facet on the subjacent radial. The interior 

 of this gap, however, is filled up by the deeper portions of the 

 oral plates, which thus bridge over the semicircular notcli on 

 the upper surface of the facet. The latter consists of nothing 



